Bilateral back extensor exosuit (BBEX) for multidimensional force assistance and prevention of spinal overuse injuries during lifting

Lumbar spine injuries resulting from heavy or repetitive lifting remain a prevalent concern in workplaces. Back-support devices have been developed to mitigate these injuries by aiding workers during lifting tasks. However, existing devices often fall short in providing multidimensional force assistance for asymmetric lifting - an essential feature for practical workplace use. Additionally, validation of device safety across the entire human spine has been lacking. This paper introduces the Bilateral Back Extensors Exosuit (BBEX), a robotic back-support device designed to address both functionality and safety concerns. The design of the BBEX draws inspiration from the anatomical characteristics of the human spine and back extensor muscles. Employing a multi-degree-of-freedom architecture and serially connected linear actuators, the device's components are strategically arranged to closely mimic the biomechanics of the human spine and back extensor muscles. To establish the efficacy and safety of the BBEX, a series of human-subject experiments were conducted. Eleven healthy male participants engaged in symmetric and asymmetric lifting tasks while wearing the BBEX. The results confirm the ability of the BBEX to preserve the natural range of motion of the back and to provide effective multidimensional force assistance. Moreover, comprehensive safety validation was achieved through analyses of muscle fatigue in the upper and the lower erector spinae muscles, as well as mechanical loading on spinal joints during both lifting scenarios. By seamlessly integrating functionality inspired by human biomechanics with a focus on safety, this study offers a promising solution to address the persistent challenge of preventing lumbar spine injuries in demanding work environments.

Methods
Eleven men who had no histories of low back injury or musculoskeletal disorders volunteered to participate in the study. The mean (SD in parenthesis) of age, height, sitting height and body mass of the participants were 25.73 years (0.96 years), 174.45 cm (3.06 cm), 89.91 cm (2.57 cm) and 76.36 kg (8.41 kg), respectively. Further details on the participants are described in table S2. All participants received an information letter prior to the experiments and signed an informed consent form.

All participants performed two types of tasks: a symmetric lifting task and an asymmetric lifting task, and conducted each task twice, with and without the BBEX. Thus, a total of four experiments were conducted for each participant. Each experiment was performed at intervals of one week to exclude the effects of the previous experiment. All experimental procedures performed in the study were approved by the Institutional Review Board of Seoul National University (IRB No. 1912/001-003), and all participants were familiarized with the experimental procedures and the BBEX prior to the experiments. In the preparation process, EMG electrodes and reflective markers were attached to the participants, and a heart rate monitor armband was placed on their right upper arm. For the experiments with the device, the BBEX was fitted and adjusted to the individual participants. Subsequently, reflective markers were attached to the suit as well. After the preparation, the participants took a five-minute break (58) to relieve any fatigue that might occur during the preparation process. The participants lowered and lifted a weight of 5.5 kg 30 times with an interval of 8 seconds for each experiment. In the symmetric lifting task, the participants performed the cyclic lifting task in the sagittal plane to touch a pad in front of the foot. In the asymmetric lifting task, the participants performed the lifting by rotating the trunk alternately left and right side to touch a pad, which was located +-45 degrees to the sagittal plane. We provided auditory feedback using a metronome to control lifting speed, and the participants were asked to handle the weight at a constant speed as smoothly as possible. The weight and the lifting interval were chosen to follow the limits suggested by the National Institute for Occupational Safety and Health Guidelines while guaranteeing 30 continuous lifts (59). Since the lower back extensor muscles become silent in the upright posture, we limited the maximum range of extension during the upward-lifting phase to induce sustained muscle contraction. To prevent the involvement of the lower extremities, participants were asked not to bend their knees and were instructed to keep the weight within the marked area on the floor. The ground level of the floor on which the individual participants put their feet was adjusted using a platform in consideration of the participants' range of motion (see Movie S7 for the experiment description).